1. Field of the Invention
The present invention relates to an object information acquiring apparatus.
2. Description of the Related Art
A photoimaging apparatus for obtaining information concerning the inside of an object such as a living organism using light has been studied in the medical field. As one of such techniques, there is photoacoustic tomography (PAT). This is a technique for visualizing information related to an optical characteristic value inside an object on the basis of an acoustic wave generated from a biological tissue, which absorbs the energy of light propagated and diffused in the object. As an example of an information acquiring method, there is a method of detecting acoustic waves in a plurality of places surrounding an object and mathematically analyzing an obtained signal.
Information such as an initial sound pressure distribution and light energy absorption density obtained by the technique can be used for, for example, specifying a position of a malignant tumor involving multiplication of new born blood vessels. In the following explanation, description of the light energy absorption density distribution is omitted. However, the light energy absorption density distribution is considered to be the same as the initial sound pressure distribution. A visualized image (e.g., a three-dimensional reconstructed image) is useful for grasping the inside of a living organism in a medical diagnosis.
On the other hand, according to the progress of an information processing apparatus and the increase in a data capacity in recent years, a frequency of use of a three-dimensional medial image of a human body obtained by a CT or an MRI is increasing in a diagnosis in the medical field. In general, three-dimensional image data for a medical image diagnosis includes a form image and a function image. The form image indicates an image in which a characteristic of a form of an object is shown and that is excellent in display of anatomical information like a CT image. The function image is an image that is excellent in display of physiological information like a positron emission tomography (PET) image. An image obtained by the PAT is generally classified into the function image.
A photoacoustic effect and a characteristic of a reconstructed image in the PAT are explained. The photoacoustic effect is a phenomenon in which, when light such as pulse light is irradiated on an object, volume expansion occurs in a region with a high absorption coefficient and an acoustic wave (a compressional wave called photoacoustic wave; typically an ultrasound wave) is generated.
Theoretically, in the PAT, if a temporal change of the photoacoustic wave is measured by an ideal acoustic wave detector (wideband/dot detection) at various points of a closed space surface (in particular, a spherical measurement surface) surrounding the entire object, it is possible to completely visualize an initial sound pressure distribution caused by the light irradiation. It is know that, even if a space is not a closed space, if the temporal change can be measured in a columnar shape or a flat shape with respect to the object, it is possible to substantially reproduce the initial sound pressure distribution caused by light irradiation (see PHYSICAL REVIEW E 71, 016706(2005)).
The following Expression (1) is a partial differential equation called photoacoustic wave equation. If this expression is solved, it is possible to describe acoustic wave propagation from an initial sound pressure distribution and theoretically calculate in which place and how a photoacoustic wave can be detected.
                    [                  Math          .                                          ⁢          1                ]                                                                                  (                                                            ∇                  2                                ⁢                                  -                                      1                                          c                      2                                                                                  ⁢                                                ∂                  2                                                  ∂                                      t                    2                                                                        )                    ⁢                      p            ⁡                          (                              r                ,                t                            )                                      =                              -                                          p                0                            ⁡                              (                r                )                                              ⁢                                    ∂                              δ                ⁡                                  (                  t                  )                                                                    ∂              t                                                          (        1        )            
where r represents position, t represents time, p(r, t) represents temporal change of sound pressure, p0(r) represents initial sound pressure distribution, c represents sound speed, and δ(t) represents delta function representing the shape of a light pulse.
On the other hand, the image reconstruction in the PAT is to derive the initial sound pressure distribution p0(r) from sound pressure pd(rd, t) obtained at a detection point and is mathematically called inverse problem. A universal back projection (UBP) method, which is a representative image reconstructing method, is explained. By analyzing the photoacoustic wave equation of Expression (1), it is possible to accurately solve the inverse problem for calculating p0(r). UBP is representation on a time domain of a result obtained by solving the inverse problem. Expression (2) is finally derived.
                    [                  Math          .                                          ⁢          2                ]                                                                                  p            0                    ⁡                      (            r            )                          =                              -                          2                              Ω                0                                              ⁢                      ∇                          ·                                                ∫                                      S                    0                                                  ⁢                                                                            n                      ⋒                                        0                    S                                    ⁢                                      ⅆ                                                                                            S                          0                                                ⁡                                                  [                                                                                                                    p                                0                                                            ⁡                                                              (                                                                                                      r                                    0                                                                    ,                                  t                                                                )                                                                                      t                                                    ]                                                                                            t                        =                                                                                                        r                            -                                                          r                              0                                                                                                                                                                                                                                                            (        2        )            
where Ω0 represents solid angle of an overall measurement area S0 with respect to an arbitrary reconstruction voxel (or focus point).
When the expression is plainly transformed, the following Expression (3) is obtained:
                    [                  Math          .                                          ⁢          3                ]                                                                                  p            0                    ⁡                      (            r            )                          =                              ∫                          Ω              0                                ⁢                                    b              ⁡                              (                                                      r                    0                                    ,                                      t                    =                                                                                        r                        -                                                  r                          0                                                                                                                                          )                                      ⁢                                          ⅆ                                  Ω                  0                                                            Ω                0                                                                        (        3        )            
where b(r0, t) represents projection data and dΩ0 represents solid angle of a detector dS0 with respect to an arbitrary observation point P. The initial sound pressure distribution p0(r) can be obtained by back-projecting the projection data according to the integral of Expression (3).
Note that b(r0, t) and dΩ0 are represented by the following Expressions (4) and (5):
                    [                  Math          .                                          ⁢          4                ]                                                                      b          ⁡                      (                                          r                0                            ,              t                        )                          =                              2            ⁢                          p              ⁡                              (                                                      r                                          0                      ⁢                                                                                                                            ,                  t                                )                                              -                      2            ⁢            t            ⁢                                          ∂                                  p                  ⁡                                      (                                                                  r                        0                                            ,                      t                                        )                                                                              ∂                t                                                                        (        4        )                                          d          ⁢                                          ⁢                      Ω            0                          =                                            ⅆ                              S                0                                                                                                      r                  -                                      r                    0                                                                              2                                ⁢          cos          ⁢                                          ⁢          θ                                    (        5        )            
where θ represents angle formed by the detector and the arbitrary observation point P.
When the distance between a sound source and a measurement position is sufficiently large compared with the size of the sound source (far sound field approximation), the following Expression (6) is obtained. In the expression, b(r0, t) is represented by the following Expression (7):
                    [                  Math          .                                          ⁢          5                ]                                                                      p          ⁡                      (                                          r                0                            ,              t                        )                          ⪡                  t          ⁢                                          ⁢                                    ∂                              p                ⁡                                  (                                                            r                      0                                        ,                    t                                    )                                                                    ∂              t                                                          (        6        )                                          b          ⁡                      (                                          r                0                            ,              t                        )                          =                              -            2                    ⁢          t          ⁢                                                    ∂                                  p                  ⁡                                      (                                                                  r                        0                                            ,                      t                                        )                                                                              ∂                t                                      .                                              (        7        )            
In this way, it is known that, in the image reconstruction of the PAT, the detection signal p(r0, t) obtained by the detector is subjected to time derivative to obtain the projection data b(r0, t) and the projection data b(r0, t) is back-projected according to Expression (3) to calculate the initial sound pressure distribution p0(r) (see PHYSICAL REVIEW E 71, 016706(2005))
However, Expression (1), which is the photoacoustic wave equation, used for calculating Expression (3) assumes “fixed sound speed”, “measurement from all directions”, “impulsive photoexcitation”, “acoustic wave detection in a wideband”, “acoustic wave detection at points”, and “continuous sampling of acoustic waves”. Realistically, it is not easy to realize an apparatus that satisfies these assumptions.
For example, realistically, it is difficult to detect an acoustic wave while surrounding an entire object. To increase a measurement area for an acoustic wave, it is necessary to increase the size of an acoustic wave detector and the number of elements and reinforce signal processing control. As a result, manufacturing costs increase. Because of such circumstances, a practical measurement apparatus often detects, from a specific direction, an acoustic wave from an object using a probe having a limited size.
As an example of the practical apparatus, an apparatus including a flat measurement system is devised (U.S. Pat. No. 5,840,023). The apparatus irradiates light on an object surrounded by flat plates, detects an acoustic wave with an acoustic wave detector arranged on the flat plate, and obtains a function image through image reconstruction. The apparatus can also calculate oxygen saturation on the basis of the detected acoustic wave. The oxygen saturation is content of oxygenated hemoglobin in total hemoglobin in blood. The oxygen saturation can be an index for determining whether the cardiopulmonary function is normally working and an index for determining malignity or benignancy of a tumor.
The calculation of oxygen saturation by the PAT is described in, for example, Japanese Patent Application Laid-open No. 2011-177496. Near infrared light is used for measurement of oxygen saturation. The near field light is easily transmitted through water constituting most of a living organism but is easily absorbed by hemoglobin in blood. Oxygenated hemoglobin and reduced hemoglobin respectively have different optical absorption spectra. Therefore, by irradiating the near infrared light on the living organism, it is possible to image a hemogram, which is form information of the living organism, and calculate an oxygen saturation value. Specifically, photoacoustic measurement is performed using near infrared light having different wavelengths and a comparison operation of a calculated optical absorption coefficient is performed. It is expected that diagnosis accuracy for breast cancer and the like is improved by displaying the calculated oxygen saturation value in addition to the image of the hemogram, which is the form information.    Patent Literature 1: U.S. Pat. No. 5,840,023    Patent Literature 2: Japanese Patent Application Laid-Open No. 2006-023820    Patent Literature 3: Japanese Patent Application Laid-open No. 2011-177496    Non Patent Literature 1: PHYSICAL REVIEW E 71, 016706(2005)